Implement LIBDPLIBS, which is a list of the tuples:
libname path-to-srcdir-of-libname
For each tuple;
* LIBDO.libname contains the .OBJDIR of the library `libname', and
if it is not set it is determined from the srcdir and added to
MAKEOVERRIDES (the latter is to allow for build time optimization).
* LDADD gets -L${LIBDO.libname} -llibname added.
* DPADD gets ${LIBDO.libname}/liblibname.so added.